Even in a 401(k) plan, a spousal waiver would have to be witnessed by either a notary or a plan representative before it is valid. If you haven't done that and you were married for at least a year, you are his beneficiary. If you were married less than a year, the plan document will determine if you are treated as a surviving spouse or not.
My suggestion is to send the plan sponsor a letter telling them you are his surviving spouse and that your letter is a claim for the death benefits from his account. Include a copy of your marriage license, if you have it available. If they think you are not entitled to receive the benefits, they are supposed to explain why, including cites to appropriate sections of the plan document.